1. Field of the Invention
The present invention relates to a liquid crystal display device used as a display for personal computers, word processors, amusement equipment, TV apparatuses, etc. In particular, the present invention relates to a liquid crystal display device in which liquid crystal molecules in a liquid crystal layer are axially symmetrically aligned in display pixels.
2. Description of the Related Art
As means for improving viewing angle characteristics of a liquid crystal display device, Japanese Laid-Open Publication No. 8-278504 discloses a liquid crystal display device in which liquid crystal molecules are axially symmetrically aligned in display pixels.
Hereinafter, the above-mentioned liquid crystal display device will be described with reference to FIGS. 8A and 8B. FIG. 8A is a plan view of the liquid crystal display device, and FIG. 8B is a cross-sectional view thereof.
In the above-mentioned liquid crystal display device, an active matrix substrate 401a includes a glass substrate 301a on which transparent electrodes 302a are provided. A first insulating layer 310a made of black resin is patterned on the transparent electrodes 302a so as to surround display pixels, and a second insulating layer 310b in a column shape is formed on the first insulating layer 310a. The second insulating layer 310b is formed by spin-coating a negative photoresist mixed with 0.1% by weight of spacers, and exposing the photoresist to light through a mask, followed by development and patterning.
The active matrix substrate 401a is attached to a counter substrate 401b in which a transparent electrode 302b is provided on a glass substrate 301b. Thus, a liquid crystal panel is constructed. Herein, spacers for keeping a cell gap between the substrates are fixed outside display pixels by the resist, and are not present in the display pixels.
A liquid crystal layer is interposed in the gap between the substrates, which includes liquid crystal regions 303 in the display pixels and polymer regions 304 outside the display pixels. The liquid crystal layer is produced as follows: a mixture of a polymerizable resin material and a liquid crystal material having positive dielectric anisotropy is injected into the liquid crystal panel, and UV-rays are irradiated to the mixture under the application of a voltage of about 2.5 volts at about 60 Hz and a temperature which renders the mixture uniform, whereby the resin is cured. After being cooled to about 40.degree. C. over 5 hours under the application of a voltage, the liquid crystal panel is returned to room temperature, and the resin is completely polymerized by UV irradiation. As a result, the polymerizable resin material is preferentially separated on the insulating layer, whereby the polymer regions 304 are formed outside the display pixels. The liquid crystal regions 303 in the display pixels are in a mono-domain state, and the liquid crystal molecules are axially symmetrically aligned therein.
In the above-mentioned liquid crystal display device, liquid crystal molecules are axially symmetrically aligned with respect to a central portion of a domain when no voltage is applied. Under the application of a voltage, liquid crystal molecules rise in an axially symmetrical direction. Accordingly, an apparent refractive index seen in each direction is rendered uniform, which improves viewing angle characteristics.
Another method is also disclosed, for mask-printing a resist material mixed with spacers onto a substrate to provide the spacers at predetermined positions. Furthermore, a structure in which a resist without being mixed with spacers may be provided up to the same height as that of a cell gap, whereby the cell gap is maintained only by the resist. Furthermore, there is disclosed a method for fixing spacers at predetermined positions by coating and patterning a resist, dispersing spacers in a dry state, and further coating and patterning a resist. Furthermore, a method for fixing spacers at predetermined positions in display pixels by a resist is also disclosed.
According to the above-mentioned prior art, after a resist material mixed with spacers is spin-coated onto a substrate, the resist material is baked and exposed to light through a mask, followed by development, whereby the spacers are fixed at predetermined positions. In this case, when the substrate is soaked in a developer to develop the resist, the spacers mixed with the resist enter the developer together with the resist. Therefore, the developer is contaminated, which necessitates the use of a larger amount of developer.
On the other hand, according to the method for mask-printing a resist material mixed with spacers onto a substrate using a screen plate so as to fix the spacers at predetermined positions, it is difficult to print the resist at a minute pitch.
Alternatively, in the case of keeping a cell gap only by a resist without being mixed with spacers, the resist is required to be provided up to the same height as that of the cell gap. Thus, the volume of the resist contained in a liquid crystal panel becomes larger, compared with the case where a resist material is mixed with spacers. Some resin material and resist material are unlikely to discharge all the air adsorbed thereto or contained therein. Therefore, in the case where liquid crystal is injected into a liquid crystal panel containing a large amount of resist by a vacuum injection method, the deaeration time of the liquid crystal panel becomes long depending upon the kind of material.
Furthermore, according to the method for coating and patterning black resin, dispersing spacers in a dry state, and further coating and patterning a resist to fix spacers at predetermined positions, the number of steps is large.
According to the above-mentioned prior art, a method for fixing spacers at predetermined positions in display pixels by a resist is disclosed. However, in the case where a plurality of spacers are provided in one display pixel, the following problem arises.
In the case where liquid crystal molecules are axially symmetrically aligned, axially symmetrical alignment is likely to be formed with respect to an uneven portion of a substrate. Therefore, the liquid crystal molecules are likely to be axially symmetrically aligned with respect to the positions at which spacers are provided. In the case where a plurality of spacers are present in one display pixel, a plurality of centers for axial symmetry are generated in one display pixel.
When the liquid crystal molecules are axially symmetrically aligned, a rising direction of the liquid crystal molecules around the center for axial symmetry changes by 360.degree.. Therefore, in the case where one center for axial symmetry is present in a display pixel, viewing angles seen in each slanting direction compensate for each other, whereby wide viewing angle characteristics can be obtained. However, in the case where a plurality of centers for axial symmetry are generated caused by spacers present irregularly in one display pixel, an apparent refractive index seen in each direction is not rendered sufficiently uniform. Therefore, roughness is observed in a slanting direction, and a display quality is decreased.
In the case where spacers are not present at central portions of display pixels, the effect of compensating for viewing angle is decreased. Therefore, roughness is observed in a slanting direction, and the display quality is decreased.